Electrodeposition, although known for some time, has only recently become of commercial importance as a coating application method. Along with the increased use of such methods has been the development of certain compositions which can provide satisfactory coatings when applied in this manner. While many compositions can be electrodeposited, most coating compositions when applied using electrodeposition techniques do not produce commercially usable coatings. Moreover, electrodeposition of many coating materials, even when otherwise successful, is attended by various disadvantages such as non-uniform coatings and by poor throwing power. In addition, the coatings obtained are in most instances deficient in certain properties essential for their utilization in many applications for which electrodeposition is otherwise suited. In particular, properties such as corrosion resistance and alkali resistance are difficult to achieve with the resins conventionally employed in electrodeposition processes. This is especially true with the conventional electrodeposition vehicles, which contain polycarboxylic acid resins solubilized with a base; these deposit on the anode and because of their acidic nature tend to be sensitive to common types of corrosive attack, e.g., by salt, alkali, etc. Many electrodeposited anodic coatings are subject to discoloration or staining because of dissolution of metal ions at the anode.
Cationic resins have been developed which are the most useful resins for many purposes and have excellent corrosion resistance and other properties, for example, those disclosed in copending applications Ser. Nos. 772,353, filed Oct. 31, 1968, now U.S. Pat. No. 3,619,398, and 100,834, filed Dec. 22, 1970, now abandoned.
However, both anionic and cationic resins utilized by the art in electrodeposition are solubilized with a counter-ion, that is, a polyacid resin to solubilize with a base and likewise the polybasic resins are solubilized with an acid. During the electrodeposition, when the resin is coated, the counter-ion is dissociated and becomes free in the bath. The control of this counter-ion which ultimately affects bath and coating properties when it accumulates, has presented a serious and costly problem to the industry.
It has now been found that cationic resins containing a Zwitterion, which are internally solubilized, need no additional counter-ions and thus eliminate a major control problem in the electrodeposition system.